1. Field of the Invention
The invention relates to a method of fabricating a solar module with structured and integrated series-connected thin-film solar cells and to solar modules made by the method. The support layer of such solar cells may be either a substrate or a superstrate.
Thin-film solar cells of either type are provided with light-absorbing absorber layers of cost-efficient amorphous, poly- or micro-crystalline semiconductor materials which may be precipitated or built up on large-surface substrates or superstrates by a many different methods. The small layer thickness of the absorber layers and the possibility of providing a structure during fabrication further reduce the manufacturing costs, so that thin-film solar cells constitute a cost-efficient alternative to the cost-intensive silicon solar cells mostly used at present and which as mono-crystalline single or multiple layer systems following their manufacture must first be sawed apart into individual cells and then, in the manner of high-value semiconductor products, be further processed by complex steps. By photovoltaic conversion of solar energy into electric power, thin-film solar cells generate voltage levels of less than 1 Volt. In order to attain technically useful power at a voltage of typically 12 Volts or 24 Volts, a sufficient number of individual cells are connected in series. In the case of thin-film solar cells, the series connection may be integrated into the layer-forming process. This involves subdividing layers formed as whole surfaces into small strips by suitable structuring methods, for instance paste-scribing methods and lift-off techniques as well as mechanical and, more particularly, layer processing methods. The purpose of the structuring is to create an electrical connection between the electrodes at the front and rear surfaces of adjacent strip-shaped solar cells.
2. The Prior Art
From U.S. Pat. No. 4,675,467 there is known a method of series-connecting an integrated thin-film solar module in which both electrodes are incorporated in prefabricated strip-form into an unstructured absorber layer. The conductive connections between the corresponding electrodes of adjacent solar cells are then formed in a structuring step by laser irradiation from the transparent substrate surface into an area covered by the electrode strips. Appropriate areas of the absorber layer are thus converted into low-ohmic areas by a precisely defined quantity of energy, which does, however, entail the risk of damaging the semiconductor material. Because of the lack of spatial structuring of the absorber layer there is no electrical insulation between the semiconductor material of adjacent solar cells which leads to power-reducing short circuit currents. The laser treatment requires a highly precise power level, positioning and focusing of the applied laser beam in order to yield the desired spatially precise conversion effect. This may lead to layer separations and damages in the immediate vicinity of the structuring operation. Furthermore, it is necessary always to use a transparent substrate of precisely defined homogeneous layer thickness so that the laser beam may penetrate from the substrate side and that a power-level dependent depth of penetration into the layers to be separated or transformed may be precisely set.
A similar process involving prefabricated electrode strips has been described in U.S. Pat. No. 4,999,308, in which the laser treatment for area conversion is used at the same time to separate the absorber layer to form insulation trenches by blowing away semiconductor material which is thus lost. In this combined process, the energy dosaging poses a problem which to some extent leads to an uncertainty especially in respect of the conversion areas, notwithstanding the fact that processing is carried out from the upper side of the solar cells rather than through the substrate. The use of two scribing processes for the consecutive separation of absorber layer and front electrode at different laterally displaced positions is known from U.S. Pat. No. 5,296,674. The separation is accomplished by indirect laser irradiation through the protective layer substrate so that the direct connection between adjacent solar cells is maintained by the absorber layer. This method requires multiple positioning of a transparent substrate while accepting short circuit currents.
A method of series connecting an integrated thin-film solar cell module is known from WO-9503628 in which all functional layers are separately structured during special process steps. In accordance with the method, a metal layer previously precipitated as a single layer on a transparent substrate is separated by any desired structuring method into closely adjacent strips to form a strip-shaped rear electrode. Following the subsequent whole-surface coating with a thin semiconductor layer for forming an absorber layer and with a front layer to form a front electrode, two additional separate structuring steps are carried out by laser irradiation from the side of the substrate. The first laser irradiation serves to structure or pattern the strips of the absorber layer and of the front electrode; the second laser irradiation serves to convert into a low-ohmic area that portion of the absorber layer which is positioned in the covered area between opposite electrode strips of adjacent solar cells, thus forming an integrated conductive series-connection between the solar cells. Therefore, in the known method the structuring involves a treble separation treatment. The first treatment serves to separate the front electrode and the absorber layer. Laser-assisted removal of the sensitive semiconductor layer always involves the risk of damage to, or alteration of, the layer. The second treatment for area conversion again requires a precisely energized laser and includes the problems described supra.
The described methods are footed on the common task of optimization in the sense of maximized power output or minimized surface area of the fabricated solar modules of striped structured thin-film solar cells of either type of support layers. Compared to monocrystalline solar cells such solar cells are of low energy efficiency which at lower light conditions is rapidly further reduced relative to normal conditions (light concentration AM 1.5). Accordingly, at common deviations of seasonal and day-by-day light intensities, depending upon the weather and at indoor applications (down to 10% of the maximum available radiation), thin-film solar cells are subject to significant power losses. This explains the insignificant use of thin-film solar cells in areas where solar radiation varies significantly and in indoor areas in general. In connection with monocrystalline solar cells, light collecting concentrator modules consisting of optical elements to maintain at, or raise to, an optimum efficiency range the light intensity in respect of monocrystalline solar cells have become known from many publications. However, the goal of such known measures is not to maximize power but, rather, to bring about as significant a reduction as possible of the very expensive required surface of the solar module.
For instance, U.S. Pat. No. 5,118,361 discloses a solar module of monocrystalline tandem solar cells made of GeAs/GaSb which is built into a housing the cover of which is formed by a concentrator module made up of individual Fresnel lenses of polymeric material and positioned, together with light-collecting funnels, in front of individual solar cells. These are disposed within the module on a flexible connection ribbon of conductive and non-conductive strips. From European Patent 0,657,048 there is known an automated microchip-like connection in a very similar arrangement of monocrystalline GaAs solar cells aiming at surface minimization. Concentrator arrangements provided with linear focus lenses in, or as part of, a module cover which are particularly suitable for strip-like solar modules are known, for instance, from U.S. Pat. No. 4,711,972 for monocrystalline silicon solar cells and from U.S. Pat. No. 5,505,789 for monocrystalline integrated solar cell chips made of GaAs. German Patent specification 197 44 840 A1 discloses a solar module having a forward concentrator module made of plastic Fresnel lenses which as a structured unit tilts or slides in accordance with the position of the sun to provide for an improved power balance. Finally, European Patent 0,328,053 describes strip-like solar modules with a forward Fresnel lens which are mounted in the corner of a window pane of a double window and which are to provide the electricity for operating blinds in the between the windows.
However, neither the mentioned nor any other publication discloses an application for concentrator modules for amorphous, poly- or micro-crystalline thin-film solar modules in particular in any kind of arrangement, so that such modules have hitherto shown a relatively poor power balance which is strongly dependent on the time of day and on the weather. More particularly, little or no consideration has been given to optical structuring methods in connection with applications of known solar modules, including thin-film solar cells precipitated as large surfaces on glass substrates, which has led to relatively uniform solar module structures used primarily in the field of industry. Esthetically patterned solar modules may be found, for instance, in roof pans (see German Patents 42 279 29 and 43 176 74) or wrist watches which also may take different colors into consideration (see European Patent 0,895,141).